Considered in particular is a rocket engine combustion chamber extending in a longitudinal direction defined by its axis of symmetry, the combustion chamber therefore being substantially rotationally symmetrical. The axis of symmetry is therefore contained in the combustion chamber, unlike cases of annular combustion chambers. In such combustion chambers, the propellants (fuel and oxidizer fluid, for example liquid hydrogen and liquid oxygen) are injected at one end 11 of the chamber 10 by injectors. FIG. 1 shows such a combustion chamber 10. The combustion reaction of the propellants produces combustion gases (for example water vapor) that are expelled by a neck 15 situated opposite the injectors. Downstream of the neck 15 (location of the combustion chamber with the smallest section), the chamber flares by a divergent section 20, which makes it possible to increase the speed of the combustion gases expelled through the neck 15, and therefore the thrust delivered by the engine. This divergent section 20 of the chamber 10 extends downstream via a divergent section 80 of the rocket engine. This divergent section 80 is fixed to the downstream end 25 of the divergent section 20 of the chamber 10, and is a separate part of the rocket engine from the combustion chamber 10.
The walls of the combustion chamber 10 are typically made from copper or a copper alloy, which offers the best compromise between thermomechanical resistance and thermal conductivity. During the operation of the rocket engine, these walls, including the wall 30 of the divergent section 20, are brought to very high temperatures (the combustion gases can be at a temperature in the vicinity of 3500 K upstream of the neck in the case of oxygen and hydrogen combustion) and must be cooled (their temperature at the neck must not exceed 1000 K) in order to keep their mechanical properties. The most common method for performing this cooling consists of circulating one of the propellants in or in contact with the wall 30 of the divergent section 20 of the chamber 10 because these propellants are at a very low temperature.
In fact, the propellants currently used are liquefied gases (to minimize their volume), and are therefore at a very low temperature when they are injected into the combustion chamber 10. As a result, these propellants circulating (before their injection) around the wall 30 of the divergent section 20 are at a temperature (20 K to 100 K) that is much lower than the ambient temperature. Ambient temperature refers to a temperature of about 300 K.
This propellant circulation makes it possible to cool the wall 30 such that, during operation of the engine, the temperature of the inner face 32 of the wall 30 is lower than the condensation temperature of the combustion gases escaping through the divergent section 20. For example, the temperature of the inner face 32 is less than 400 K, for example less than 300 K. As a result, the combustion gases (water vapor in the case of an oxygen-hydrogen combustion) circulating in the divergent section 20 along the wall 30 condense on the inner face 32 of said wall 30, which is undesirable.
Indeed, this condensation causes streaming along this inner face 32, which disrupts the flow of the combustion gases. Moreover, this condensation locally causes variations in the temperature of the inner face 32, which locally generates important stresses that can lead to a decrease in the lifetime of the chamber. It is therefore necessary to increase the temperature of the inner face 32 of said wall 30 in order to eliminate that condensation.
One solution considered to increase the temperature of said inner face 32 consists of increasing its roughness so as to increase the exchange surface of said inner face 32 with the combustion gases (water vapor in the case of an oxygen-hydrogen combustion) of the inside of the chamber. However, this solution is difficult to carry out because said roughness must be very fine for the exchange surface to be increased enough. Furthermore, modeling heat exchanges is very complex in the case of such a rough surface.